Sensor device

ABSTRACT

The present invention relates to a sensor device and method for measuring a property associated with the introduction of or changes in a chemical, biological or physical stimulus in a localised environment, in particular to a sensor device and method for measurign the partition coefficient of a chemical stimulus such as a pharmaceutical compound.

[0001] The present invention relates to a sensor device and method formeasuring a property associated with the introduction of or changes in achemical, biological or physical stimulus in a localised environment, inparticular to a sensor device and method for measuring the partitioncoefficient of a stimulus.

[0002] There are numerous industries in which the potential utility of asystem under development can be assessed by examining thephysicochemical behaviour of the components of the system. However,measuring a physicochemical property of a material using conventionaltechniques can be both time consuming and inaccurate. In particular,properties such as the physical swelling of a material of interest (as aconsequence of vapour sorption or thermal changes for example) and thepartitioning of a material of interest between specific media tend to bemeasured quantitatively using indirect means.

[0003] By way of a specific example, the partition coefficient of amaterial of interest is generally determined manually which is both timeconsuming and expensive. Where the material of interest is a new drugcompound for example, the partition coefficient is a key quantitativeindicator of its potency. If a drug compound will not pass efficientlyacross biological membranes and through biological media, it is unlikelyto be therapeutically effective. One specific parameter which is used todetermine the potential for candidate materials to move through suchmedia is the octanol-water partition coefficient (the “logP value”). ThelogP value is a measure of the equilibrium concentration of a givenmaterial between octanol and water. The determination of the logP valueis generally carried out in accordance with the prior art as follows:

[0004] 1. octanol and water (which are immiscible) are shaken vigorouslyfor an extended period of time until the concentration of the candidatedrug compound is in equilibrium between the octanol phase and theaqueous phase;

[0005] 2. the two immiscible components are allowed to separate onstanding;

[0006] 3. aliquots of the octanol phase and the aqueous phase(separately) are withdrawn and the concentrations in each phase aredetermined using traditional analytical techniques such as GC, HPLC orthe like;

[0007] 4. the ratio of the concentration of the candidate drug compoundbetween the two phases provides the logP value which is used to deducethe potential solubility of the candidate drug compound in biologicalmedia.

[0008] From the above description, it will be apparent that the processof determining the logP value is lengthy. Given the increasing use ofcombinatorial techniques by pharmaceutical companies, there are millionsof new candidate drug compounds being generated on a monthly basis.There is therefore an increasing demand for a streamlined process fordetermining logP values to assist the drug screening program.

[0009] A sorbtion sensor with an appropriate coating may be used todetermine partition coefficients. For example, a sorbiton sensor with ahydrocarbon polymer coating (eg polyisobutylene) could be used in placeof the manual technique described above. An aqueous solution containingthe candidate drug compound could be passed directly over the sorbitonsensor and a certain amount of the candidate drug compound will bepartitioned into the polymer film causing a change in the refractiveindex of the coating. The extent of partitioning will be determinedthermodynamically and can be directly related to the logP value.Although the large surface area:volume ratio of the sorbiton sensorensures rapid equilibration and measurement, the quantitativeinformation is unreliable due to the multiple factors contributing tothe changes on the sensor surface.

[0010] Membranes are used in a wide range of well established medicalapplications. The ability of a candidate membrane material to partitionkey components in biological fluids is a key indicator of theirpotential as effective membranes. In general, there is a need for areliable and rapid means of determining partitioning behaviour to aidthe development of membrane materials.

[0011] Partitioning is also a key indicator in the development of newwound dressings. If certain key proteins move from a patient to a testmaterial present in a wound dressing, the likelihood of the testmaterial sticking to the wound or supporting deleterious material may beincreased. Similarly, the partitioning of certain materials away fromthe area of the wound may be advantageous. Thus the partitioningbehaviour of candidate materials for wound dressings is an importantparameter in the industry.

[0012] A further example of partitioning behaviour as a key indicator isin the food industry. The movement of both gaseous and liquid materialsis of importance in determining the likely effectiveness of candidatepackaging materials. In general, it is advantageous if the candidatepackaging materials partition unwanted materials in preference to thefood material. There is a need for a reasonable and cost effectivemethod for determining partitioning behaviour to offer the packagingtechnologist an insight into the development of packaging strategies andthe regulator a means to determine the safety of food packagingmaterials.

[0013] The present invention is based on the recognition that certainsensing materials exhibit a biphasic optical response on exposure to astimulus which is attributable to compositional and dimensional factors.More particularly, the present invention provides a sensor deviceexhibiting a monophasic response which may be used to measure rapidlythe physicochemical behaviour in a localised environment in which asensing material is exposed to a stimulus. The sensor device uses theoptical properties of a specialised architecture to exhibit improvedreliability, improved signal to noise ratio (sensitivity) androbustness.

[0014] Thus viewed from one aspect the present invention provides asensor device for measuring a property of interest associated with theintroduction of or changes in a stimulus (eg a chemical, physical orbiological stimulus) in a localised environment, said sensor devicecomprising:

[0015] a sensor component including a sensing material capable ofinducing or exhibiting a measurable response to a change in thelocalised environment caused by the introduction of or changes in thestimulus, wherein the sensor device is arranged so as to expose to thelocalised environment at least a part of the sensing material andwherein the sensing material is adapted to induce or exhibit amonophasic response to the change in the localised environment caused bythe introduction of or changes in the stimulus.

[0016] In a preferred embodiment, the sensing material may be applieddirectly onto the surface of the sensor component as a sensing layer. Inthis embodiment, the sensor device is arranged so as to expose to thelocalised environment at least a part of the sensing layer and thesensing layer is adapted to induce or exhibit a monophasic response tothe change in the localised environment caused by the introduction of orchanges in the stimulus. The sensing material may be applied to thesurface of the sensor component by any conventional technique eg polymerspinning, dipping or plasma polymerisation.

[0017] Preferably the monophasic response is attributable to thedominant contribution of dimensional factors to changes in the effectiverefractive index of the sensing material (eg sensing layer).

[0018] Preferably the monophasic response is attributable to thedominant contribution of compositional factors to changes in theeffective refractive index of the sensing material (eg sensing layer).

[0019] In a preferred embodiment, the property of interest may beswelling of the sensing layer. In this embodiment, the sensing layer isadapted to substantially eliminate the contribution of compositionalfactors to the effective refractive index of the sensing material.

[0020] In a preferred embodiment, the sensor device may be used todetermine a physicochemical property of the stimulus to which thesensing material is exposed (eg its partition coefficient orconcentration). By way of example, the drug industry could utilise thesensor device of the present invention to screen candidate drugcompounds for their potential to cross biological membranes. In thisembodiment, the sensing layer is adapted to substantially eliminate thecontribution of dimensional factors (eg swelling) to the effectiverefractive index of the sensing material.

[0021] In a particularly preferred embodiment, the sensor device may beused to determine concurrently a physicochemical property of thestimulus to which the sensing material is exposed and a property of thesensing material. In this embodiment, the sensor component includes twosensing layers, a first sensing layer is adapted to substantiallyeliminate the contribution of dimensional factors to the effectiverefractive index of the sensing material and a second sensing layer isadapted to substantially eliminate the contribution of compositionalfactors to the effective refractive index of the sensing material. Thefirst and second sensing layer may (if desired) be provided on separatesensor components.

[0022] Typically the sensing layer is adapted to induce or exhibit amonophasic response by empirically selecting an appropriate thickness.For example, the film thickness of the sensing layer may be measuredagainst phase change over a range of thicknesses to identify thespecific thickness (or thicknesses) at which a monophasic responseoccurs.

[0023] The architecture of the sensor device of the invention may beorganised in such a way as to enable precise measurements to be madeeither across the entire architecture or at given locations. It may bedesigned such as to reduce contributions from the ambient environmentand unwanted ‘background’ events.

[0024] In the sensor device of the present invention, the method ofinterrogation may be evanescent field mode or whole waveguide mode. Inthe latter case, the sensing layer acts as a sensing waveguide andinterrogation is direct.

[0025] The sensor device of the invention is preferably used inevanescent mode and comprises a secondary waveguide in which the sensinglayer is capable of inducing a measurable response to a change in thelocalised environment caused by the introduction of or changes in thestimulus. In this embodiment, the sensor device is advantageouslyadapted to optimise the evanescent component so as to induce in thesecondary waveguide a measurable optical response.

[0026] Generally speaking, it is known to make use of the evanescentfield component of electromagnetic radiation incident on a waveguidestructure (ie the field which extends outside the guiding region) tosense discrete changes in optical properties (see inter aliaGB-A-2228082, U.S. Pat. No. 5,262,842, WO-A-97/12225 and GB-A-2307741).This method relies on “leakage” of optical signals from the waveguidestructure into the sensing layer. The evanescent component of theoptical signal being guided by the waveguide structure is typicallysmall leading to limited interrogation of the sensing layer. This‘leakage’ or evanescent field has well defined characteristics, rapidlydecaying as the distance from the surface of the sensor platformincreases.

[0027] Preferably the sensor device in evanescent mode comprises asecondary waveguide and a reference secondary waveguide. It is preferredthat the secondary waveguide and reference secondary waveguide haveidentical properties. Preferably the secondary waveguide comprisessilicon oxynitride or silicon nitride. The reference secondary waveguidemay comprise silicon oxynitride or silicon nitride so as to haveidentical properties to the secondary waveguide.

[0028] Preferably the (or each) waveguide of the sensor component is aplanar waveguide (ie a waveguide which permits light propagation in anyarbitrary direction within the plane). Preferably, the sensor componentof the sensor device of the invention constitutes a multi-layeredstructure (eg a laminate structure). In this sense, the sensor device issimple to fabricate and fault tolerant in terms of construction errors.In a preferred embodiment, the plurality of layers in the sensorcomponent are built onto a substrate (eg composed of silicon) throughknown processes such as PECVD, LPCVD, etc. Such processes are highlyrepeatable and lead to accurate manufacture. Intermediate transparentlayers may be added (eg silicon dioxide) if desired. Typically thecomponent is a multilayered structures of thickness in the range 0.2-10microns.

[0029] The sensor device of the invention may be operated as a surfaceplasmon resonance sensor, attenuated total reflection sensor, totalinternal reflection sensor, surface acoustic wave sensor,spectrophotometric device or interferometer. For example, aninterferometer similar to that described in WO-A-98/22807 and also inPCT/GB01/03348 may be used. Such a sensor device is simple to fabricateand fault tolerant in terms of construction errors.

[0030] The interaction of the stimulus with the sensing material may bea binding interaction or absorbance or any other type of physical orchemical interaction. The sensing material may be functionalised orcoated as appropriate for the required sensing application. Typicallythe sensing material is polymeric (eg an absorbent polymer).

[0031] In a preferred sensor device of the invention, the sensingmaterial comprises an absorbent material (eg a polymeric material suchas polysiloxane or an oligomeric material such as a long chainhydrocarbon eg a C₁₈-hydrocarbon) or a bioactive material (eg containingantibodies, enzymes, DNA fragments, functional proteins or whole cells).The absorbent material may be capable of absorbing gases, liquids orvapours containing a chemical stimulus. The bioactive material may beappropriate for liquid or gas phase biosensing.

[0032] For measuring chemical stimuli, the sensing material may beabsorbent and typically polymeric or oligomeric (eg a C₁₈-oligomer suchas a C₁₈-hydrocarbon oligomer). The oligomer may be chemically bound tothe surface of the sensor component to resist stripping or dissolvingduring use and may be short (eg C₃) to produce a thin sensing layer,long (eg C₁₈) to produce a thick sensing layer or very long (eg C₁₅₀) toproduce a very thick sensing layer. The oligomer may be chosen inaccordance with practices familiar to those skilled in the art to varythe absorbtion characteristics. For example, polyethers (such aspolyethylene oxide) will absorb a wider range of chemical stimuli than aC₁₈-hydrocarbon oligomer.

[0033] A preferred device of the invention comprises a referencematerial (eg a reference layer). The physical, biological and chemicalproperties of the reference material are conveniently as similar aspossible (eg substantially identical) to those of the sensing material(with the exception of the response to the change in the localisedenvironment caused by the introduction of or changes in the stimulus).The reference material is either untreated or inactivated with respectto the sensing mechanism utilised in the sensing material.

[0034] Preferably the sensor component includes one or more additionalsensing layers or sensing regions to enable different events atdifferent localised environments to be detected. Each sensing layer orsensing region may be provided on the same or different sensor component(eg in an array). In one embodiment, the sensor device comprises asecond sensing layer capable of inducing or exhibiting a measurableresponse to a change in the localised environment caused by theintroduction of or changes in the stimulus from which measurements ofthe thickness of the material deposited on the sensor component may bededuced. In a further embodiment, the sensor device comprises a sensingregion remote from the sensing layer which is a bare region of thesecondary waveguide upon which the sensing material is deposited. Thesensing region acts as a reference region by advantageously compensatingfor changes in the refractive index of the medium containing thestimulus to which the sensor device is subjected.

[0035] In a preferred embodiment, the sensor device comprises a secondsensor component including a reference material substantially incapableof inducing or exhibiting a measurable response to a change in thelocalised environment caused by the introduction of or changes in thestimulus, wherein the sensor device is arranged so as to expose to thelocalised environment at least a part of the reference layer of thesecond sensor component.

[0036] The first and second sensor components may be integrated ordiscrete. For example, the first and second sensor components may beintegrated onto a common substrate (a “back-to-back sensor”). In thisembodiment, the localised environment surrounds the first and secondsensor component (eg the sensor components may be typically immersed ina liquid or gas phase stimulus) so as to expose to the stimulus at leasta part of the sensing layer of the first component and at least a partof the reference layer of the second component. Alternatively forexample, the first and second sensor components may be discretely builtonto separate substrates (a “dual sensor”). In this embodiment, thelocalised environment constitutes a gap between the first and secondsensor component which the stimulus may fill so as to expose to thestimulus at least a part of the sensing layer of the first component andat least a part of the reference layer of the second component. Forexample, a spacer such as a microstructure may be positioned to providea gap between the surfaces of the first and second sensor components. Incertain cases, the surface tension in a liquid phase stimulus may besufficient to maintain the gap between the first and second sensorcomponent. The gap is typically less than 10 microns.

[0037] The sensor device may comprise one or more means for intimatelyexposing to the localised environment at least a part of the sensinglayer, said means being optionally integrated onto the sensor component.

[0038] The means for intimately exposing to the localised environment atleast a part of the sensing layer (and any additional functionality) maybe provided in a microstructure positionable on the surface of and inintimate contact with the sensor component. Preferably themicrostructure comprises means for intimately exposing to the localisedenvironment at least a part of the sensing layer in the form of one ormore microchannels and/or microchambers into which chemicals may be fed(or chemical reactions may take place).

[0039] In a preferred embodiment, the means for intimately exposing tothe localised environment at least a part of the sensing layer isincluded in a cladding layer. For example, microchannels and/ormicrochambers may be etched into the cladding layer. The cladding layermay perform optical functions such as preventing significantdiscontinuities at the boundary of the sensing layer or chemicalfunctions such as restricting access of species to the sensing layer.The cladding layer may be integrated onto the sensor component.

[0040] Preferably, the whole of or a portion of any additionalfunctionality may be included in the cladding layer. Additionally, thesensing layer may be incorporated in the cladding layer in the form ofan absorbent material. Particularly preferably, the whole additionalfunctionality may be provided in the cladding layer and include devicessuch as for example quadrature electric field tracks or othermicrofluidic devices.

[0041] Electromagnetic radiation generated from a conventional sourcemay be propagated into the sensor component in a number of ways. In thepreferred embodiment, radiation is simply input via an end face of thesensor component (this is sometimes described as “an end firingprocedure”). Preferably (but not essentially), the electromagneticradiation source provides incident electromagnetic radiation having awavelength falling within the visible range. Preferably the sensordevice comprises: propagating means for propagating incidentelectromagnetic radiation into the sensor component. For example, one ormore coupling gratings or mirrors may be used. A tapered end couplerrather than a coupling grating or mirror may be used to propagate lightinto the lowermost waveguide.

[0042] The incident electromagnetic radiation may be oriented (eg planepolarised) as desired using an appropriate polarising means. Theincident electromagnetic radiation may be focussed if desired using alens or similar micro-focussing means.

[0043] Using electromagnetic radiation of different frequencies (eithersimultaneously or sequentially) may vary the contribution of the sensorcomponent and may further enhance the utility of the device.

[0044] Multimode excitation may provide useful additional information.For example, by comparing the outer and inner areas of the interferencepattern, it may be possible to determine the extent to which anyrefractive index change has been induced by changes in the thickness ofthe outer regions and the degree to which it has been effected byphysicochemical changes in the inner regions.

[0045] Thus the sensor device comprises: first irradiating means forirradiating the sensor component with TM mode electromagnetic radiationand second irradiating means for irradiating the sensor component withTE mode electromagnetic radiation. The relative phase changes of the twomodes may be used to identify and quantify the nature of the opticalchanges taking place in the sensing layer. For example, it may bepossible to attribute changes in the effective refractive index of thesensing layer to specific changes in dimension (eg expansion orcontraction) and/or composition. The relative phase changes of the twomodes may also be used to identify such changes taking place insubsequent layers when more compact structures are employed.Conveniently, measurement of capacitance and refractive mode index ofthe two modes yields further information on changes occurring in theabsorbent layer.

[0046] Transverse electric and transverse magnetic phase shifts may becompared sequentially or simultaneously in order to resolve effectivethickness changes from changes in the intrinsic refractive index inrealtime on the sensor device.

[0047] Electromagnetic radiation may be modulated (amplitude, frequencyor phase for example) to provide additional information on the behaviourof the sensor device.

[0048] An interference pattern is generated when the electromagneticradiation from the sensor component is coupled into free space and maybe recorded in a conventional manner (see for example WO-A-98/22807). Anoptical response of the sensor component to changes in the localisedenvironment may be measured from movement of the fringes in theinterference pattern. For example, the phase shift of the radiation inthe sensor component (eg induced in the secondary waveguide relative tothe reference secondary waveguide) may be measured. In turn, inferencesabout a property of interest associated with the introduction of orchanges in a chemical, biological or physical stimulus in the localisedenvironment may be made.

[0049] The sensor device of the invention may be arranged so thatchanges in the refractive index of material in the localised environmenteffect a measurable optical response (eg a change in the transmission ofelectromagnetic radiation down the secondary waveguide in evanescentmode) which manifests itself as a phase shift. For example, changes inthe refractive index of material in the localised environment mightoccur as a consequence of a chemical reaction. The measurable response(eg phase shift) exhibited or induced in the secondary waveguide may bethe result of changes in dielectric properties (eg changes in effectiverefractive index) of the sensing layer which is attributable to thecontribution of dimensional and/or compositional factors. By way ofexample, the movement of the interference pattern may be used to deducethe phase shift which takes place in the secondary waveguide (egrelative to the reference secondary waveguide) during the passage ofelectromagnetic radiation through the sensor component. The relativephase shift is directly proportional to changes occurring in theeffective refractive index of the sensing layer due to the introductionof or changes in a chemical, biological or physical stimulus in alocalised environment.

[0050] Movement in the interference fringes may be measured either usinga single detector which measures changes in the electromagneticradiation intensity or a plurality of such detectors which monitor thechange occurring in a number of fringes or the entire interferencepattern. The one or more detectors may comprise one or morephotodetectors. Where more than one photodetector is used this may bearranged in an array.

[0051] In an embodiment of the sensor device, the electromagneticradiation source and one or more detectors are integrated with thedevice into a single assembly.

[0052] The sensor component may be excited across its width and atwo-dimensional photodiode array (or the like) may be used toeffectively interrogate “strips” of the sensor device (eg an arraysensor). This may be carried out across more than one axissimultaneously or sequentially to provide spatially resolved informationrelating to events on the surface of the sensor component.

[0053] The sensor components may be optionally perturbed (eg thermallyperturbed) to enable the sensor device to be biased. This enables theprecise degree of optical response (eg phase shift) caused by thechemical or physical stimulus to be determined.

[0054] A plurality of electromagnetic radiation detector units (eg in anarray) and/or a plurality of electromagnetic radiation sources may beused to measure in discrete areas of the sensor component simultaneouslythe responses to changes in the localised environment. Alternatively,the position of the electromagnetic radiation detector andelectromagnetic radiation source relative to the sensor component may bechanged to provide information concerning responses in discrete areas ofthe sensor component. For example, discrete responses to a change in thelocalised environment caused by the presence of the same or differentstimuli may be measured in discrete areas of the sensor component. Inthe first instance, concentration gradients of the same stimulus may bededuced. In the second instance, discrete responses in different regionsto changes in the localised environment may be measured. For thispurpose, the preferred device makes use of the versatility of theevanescent mode and comprises a plurality of separate sensing layers orregions.

[0055] Measurements may be made on a multiplicity of uniformly treatedsensor components using a one dimensional (linear) photodiode array orsingle pinhole photodiode. For example, three separate sensorcomponents, one bare, one surface functionalised and one coated to anappropriate thickness for monophasic response may be used.

[0056] Conveniently, electrodes positioned in contact with a surface ofthe sensing layer or sensing waveguide enable capacitance to be measuredsimultaneously. The electrodes may take the form of either parallelplates laid alongside the plurality of planar waveguides or as aninterdigitated or meander system laid down on the top and bottomsurfaces of the sensing waveguide or sensing layer or adjacent to it. Inthe case of a meander system, the metal forming the electrode isresponsible for absorbing excessive amounts of light and as such thecapacitance is measured on an adjacent structure which is not utilisedfor optical measurement.

[0057] The sensing layer may be deposited on a quartz crystal or surfaceacoustic microbalance. This would enable the determination of eitherswelling per unit mass or refractive index change per unit mass ofabsorbed test material.

[0058] Viewed from a further aspect the present invention provides amethod for measuring a property associated with the introduction of orchanges in a chemical, biological or physical stimulus in a localisedenvironment, said method comprising:

[0059] providing a sensor device as hereinbefore defined;

[0060] introducing or causing changes in the chemical, biological orphysical stimulus in the localised environment;

[0061] irradiating the sensor component with electromagnetic radiation;

[0062] measuring the optical response of the sensor component; and

[0063] relating the optical response to the property associated with theintroduction of or changes in the chemical, biological or physicalstimulus.

[0064] Preferably the method of the invention comprises: measuringmovements in the interference pattern; and relating the movements to theproperty associated with the introduction of or changes in a chemical,biological or physical stimulus.

[0065] Preferably the method of the invention comprises: measuring aplurality of discrete responses in different regions of the sensorcomponent.

[0066] Preferably the method of the invention is carried out inevanescent mode. Preferably multiple irradiation sources and/or multipledetectors are used.

[0067] Measurement ambiguity can be reduced by performing dual modeirradiation and measurement thereby further enhancing performance. Thusin a preferred embodiment, the method of the invention comprises:

[0068] irradiating the sensor component with electromagnetic radiationin TE mode;

[0069] irradiating the sensor component with electromagnetic radiationin TM mode;

[0070] measuring the optical response of the sensor component in TEmode; and

[0071] measuring the optical response of the sensor component in TMmode.

[0072] By utilising measurements in TE and TM mode, it may beadvantageously possible to deduce from the monophasic response one ormore additional properties associated with the stimulus (eg stimulusconcentration above the sensing material). The manner in which this maybe carried out is described elsewhere (eg in WO-A-01/36946).

[0073] Viewed from an even further aspect the present invention providesthe use of a sensor device according to the first aspect of theinvention for (1) measuring the partition coefficient of the stimulus ofinterest (eg a drug compound) or (2) measuring swelling of the sensingmaterial.

[0074] In the preferred use for measuring swelling of the sensingmaterial, the sensor device is calibrated so that the contribution ofdimensional factors to changes in the effective refractive index of thesensing layer predominates.

[0075] In the preferred use for measuring the partition coefficient of astimulus of interest (eg a drug compound), the sensor device iscalibrated so that the contribution of compositional factors to changesin the effective refractive index of the sensing layer predominates.

[0076] Viewed from an even yet still further aspect the presentinvention provides a process for manufacturing a sensor device ashereinbefore defined comprising:

[0077] (A) obtaining a sensor component including a sensing materialcapable of inducing or exhibiting a measurable response to a change inthe localised environment caused by the introduction of or changes inthe stimulus;

[0078] (B) arranging the sensor component so as to expose to thelocalised environment at least a part of the sensing material; and

[0079] (C) adapting the sensing material to induce or exhibit amonophasic response to the change in the localised environment caused bythe introduction of or changes in the stimulus.

[0080] Preferably step (C) comprises:

[0081] (C1) measuring the specific thickness or thicknesses of sensingmaterial at which a monophasic response is induced or exhibited by thesensing material in the sensor component.

[0082] Particularly preferably step (C) comprises:

[0083] measuring the thickness of the sensing layer against phase changeover a range of thicknesses; and

[0084] identifying the specific thickness (or thicknesses) at which amonophasic response occurs.

[0085] The sensor device of the invention may be usefully integratedinto existing chromatographic analysers (such as HPLC analysers) for thepurposes of chemical analysis. Refractometers are currently favoured forsuch measurements but are often found to be insensitive and temperaturedependent. The sensor device of the invention offers improvedsensitivity.

[0086] Viewed from a yet still even further aspect the present inventionprovides an analyser (eg an HPLC analyser) comprising a chromatographicseparation means and one or more sensor devices as defined hereinbefore.

[0087] In the analyser of the invention, the sensing material (eg itsthickness) may be adapted to induce or exhibit a monophasic response tothe change in the localised environment caused by the introduction of orchanges in the chemical stimulus in which the response is attributableto a dominant contribution of compositional factors to changes in theeffective refractive index of the sensing material.

[0088] Typically the sensor device is employed to carry out analysisdownstream of the chromatographic separation means. Such separation maybe carried out in the gas or liquid phase and typically thechromatographic separation means is a suitable column. The sensor devicemay be connected directly to the column.

[0089] The sensing material is absorbent and typically polymeric oroligomeric (eg a C₁₈-oligomer such as a C₁₈-hydrocarbon oligomer). Theoligomer may be chemically bound to the surface of the sensor componentto resist stripping or dissolving during use and may be short (eg C₃) toproduce a thin sensing layer, long (eg C₁₈) to produce a thick sensinglayer or very long (eg C₁₅₀) to produce a very thick sensing layer. Theoligomer may be chosen in accordance with practices familiar to thoseskilled in the art to vary the absorbtion characteristics. For example,polyethers (such as polyethylene oxide) will absorb a wider range ofchemical stimuli than a C₁₈-hydrocarbon oligomer. The sensing material(eg sensor coating) may be chosen to give a similar response todifferent analytes (the so-called “Universal HPLC detector” material).The sensing material may be applied to the surface of the sensorcomponent by any conventional technique eg polymer spinning, dipping orplasma polymerisation.

[0090] In a preferred embodiment, the one or more sensor devices andchromatographic separation means are integrated onto a common substrate(eg a silicon substrate). In this embodiment, it is preferred to makeuse of the evanescent mode.

[0091] An embodiment of the analyser of the invention comprises two ormore sensor devices downstream of the chromatographic separation means(eg column). By measuring the response of the sensor component of eachsensor device, it is advantageously possible to determine time dependentfactors such as changes in temperature or pH.

[0092] An analyser in which there is a plurality of sensor devices andchromatographic separation means on a common substrate may furthercomprise multiple fluidic pathways (eg microchambers and microchannels)and multiple electromagnetic radiation pathways.

[0093] The term “optical” used hereinbefore means radiation of anywavelength in the electromagnetic spectrum or the selective absence ofsuch radiation (as in obscuration devices).

[0094] The invention will now be described in a non-limitative sensewith reference to the accompanying Figures in which:

[0095]FIG. 1 illustrates phase shift data derived from an evanescenttype sensor device as film thickness of the sensing layer is changed;and

[0096]FIGS. 2 and 3 illustrates schematically an embodiment of thesensor device of the invention in plan and side view respectively.

[0097] An embodiment of the system is shown schematically in FIG. 2.This embodiment is an evanescent waveguide interferrometer which hasbeen designed to determine simultaneously the degree of surface coverageof a test material and the degree to which certain stimuli willpartition between the bulk of the test material and medium in which thestimuli is contained.

[0098] Plane polarised radiation is generated by a suitable source (notshown). The radiation is focussed using a lens or micro-focussing object2, oriented as desired using a polariser 3 and passed to the sensorcomponent (B). In the present embodiment the structure is fabricatedfrom a silicon substrate 4, silicon dioxide transparent waveguides 5 aand 5 b, a silicon oxynitride secondary waveguide 6 a, a siliconoxynitride reference secondary waveguide 6 b, a first sensing layers 7a, a second sensing layer 7 b and a sensing region 7 c. The excitationradiation interacts with the sensing layers and sensing region whosecharacteristics are:

[0099]7 c has no coating;

[0100]7 a is a first sensing layer of test material of sufficientthickness to elicit a monophasic response from exposure to the stimuli;and

[0101]7 b is a second sensing layer of test material of sufficientthickness to elicit a monophasic response from exposure to the stimuli.

[0102] Each of 7 a, 7 b and 7 c could be provided on three separatesensors. The purpose of 7 c is to act as a reference region compensatingfor any changes in the refractive index of the medium containing thestimulus to which the sensor is subjected. 7 a provides a measure of thedegree of swelling which occurs when the test material is subjected tothe medium containing the stimulus and for this purpose the thickness of7 a has been calibrated so that the contribution of dimensional factorsdominate the contribution of compositional factors. 7 b provides ameasure of the degree of partitioning of the stimulus between the testmaterial and the medium in which the stimulus is contained and for thispurpose the thickness of 7 b has been calibrated so that thecontribution of compositional factors dominate the contribution ofdimensional factors.

[0103] The device is optimised in order to balance the amount ofradiation entering the secondary waveguide 6 a with that entering thereference secondary waveguide 6 b. Having passed down the sensorcomponent B, the output radiation is coupled into free space thusgenerating an interference pattern 8. The pattern 8 is recorded using atwo dimensional photodetector array 9. The pattern is used to determinethe relative phase shift induced in the secondary waveguide whencompared to the reference secondary waveguide. The relative phase shiftis directly proportional to changes occurring in the sensing layers 7 aand 7 b deposited on the surface of the secondary waveguide 6 a. Thesensor device provides information which is spatially resolved on thephotodetector array 9 relating to both surface and bulk effects as wellas using sensing region 7 c to compensate for external variation in themedium of the stimulus to which the sensor device is subjected.

EXAMPLE 1

[0104] The data in FIG. 1 was derived from an evanescent wave typeinterferometric device of the type illustrated in FIG. 3 with a film ofinterest deposited on the top surface. The sensor component had thefollowing characteristics: Layer Index Thickness (μm) 5a 1.49 2 6b 1.505± 0.005 1 Sb 1.47 3 6a 1.505 − 1.500 (max − min) 1

[0105] The behaviour of a polyisobutylene (PIB n=1.50) film of varyingthickness was examined upon exposure to 50% saturated toluene (n=1.49)and cyclohexane (n=1.42) vapours.

[0106] The results are illustrated in units of relative phase changeversus film thickness. The increase in film thickness upon swellingdominates the response of the ultra-thin region of the film. In the thinfilm limit, in spite of the fact that the refractive indices of bothtoluene and cyclohexane are lower than that of PIB, a relative phaseincrease is seen. This relates to an increase in effective refractiveindex. The increase in the film thickness due to swelling dominates thedecrease in the bulk film refractive index in this thickness regime.With thicker films the negative compositional refractive index changebecomes apparent. In thicker regimes, the thickness increase effectbecomes negligible and the relative phase decrease is dominated by thebulk decrease in refractive index.

SUMMARY

[0107] There are essentially two factors contributing to changes in theeffective refractive index of the film so that at most thicknesses theresponse of the sensor device is a biphasic response. Only at certainfilm thickness will one or other of the contributory factors predominateso that a monophasic response is elicited from the sensor device. Thefilm thickness providing the monophasic response depends on the testmaterial and may be determined empirically. In the Example given, athickness typically greater than 1.6 μm exhibits a monophasic responsewhere the contribution of the compositional factors dominate thecontribution of the dimensional factors.

EXAMPLE 2

[0108] The method described in Example 1 for distinguishing dimensionaleffects from compositional effects is crucial for elucidating importantparameters to allow measurement of (for example) partition coefficients.In particular, it is important to note that if the thickness is notchosen correctly a zero phase response will be seen, leading toincorrect assumptions about the partitioning behaviour of the testmaterial.

[0109] By way of example, the following steps set out schematically hownew chemical (eg drug) compounds may be rapidly screened by a chemicalcompany to determine their partition coefficient:

[0110] (1) select a polymer to coat the surface of a sensor device ofthe type described hereinbefore

[0111] (2) calibrate the polymer coating using a series of standards todetermine the optical response at different thicknesses

[0112] (3) select a thickness which induces a monphasic response (seeExample 1)

[0113] (4) coat a sensor device of the type described hereinbefore withthe polymer at the selected thickness

[0114] (5) fit the sensor device to ancillary instrumentation to createa sensor assembly

[0115] (6) supply the sensor assembly to the chemical company

[0116] (7) the chemical company recalibrates the sensor device

[0117] (8) the chemical company runs a series of samples and theresponse of each is compared to a standard to determine the logPcoefficient.

1. A sensor device for measuring a property of interest associated withthe introduction of or changes in a stimulus in a localised environment,said sensor device comprising: a sensor component including a sensingmaterial capable of inducing or exhibiting a measurable response to achange in the localised environment caused by the introduction of orchanges in the stimulus, wherein the sensor device is arranged so as toexpose to the localised environment at least a part of the sensingmaterial and wherein the sensing material is adapted to induce orexhibit a monophasic response to the change in the localised environmentcaused by the introduction of or changes in the stimulus.
 2. A sensordevice as claimed in claim 1 wherein the stimulus is a chemicalstimulus.
 3. A sensor device as claimed in claim 1 or 2 wherein thesensing material is applied directly on the surface of the sensorcomponent as a sensing layer and the thickness of the layer is adaptedto induce or exhibit a monophasic response to the change in thelocalised environment caused by the introduction of or changes in thestimulus.
 4. A sensor device as claimed in any preceding claim whereinthe monophasic response is attributable to a dominant contribution ofdimensional factors to changes in the effective refractive index of thesensing material.
 5. A sensor device as claimed in any of claims 1 to 3wherein the monophasic response is attributable to a dominantcontribution of compositional factors to changes in the effectiverefractive index of the sensing material.
 6. A sensor device as claimedin any preceding claim wherein the property of interest is swelling ofthe sensing material, and the sensing material is adapted tosubstantially eliminate the contribution of compositional factors tochanges in the effective refractive index of the sensing material.
 7. Asensor device as claimed in any of claims 1 to 5 wherein the property ofinterest is a physicochemical property of the stimulus and the sensingmaterial is adapted to substantially eliminate the contribution ofdimensional factors to changes in the effective refractive index of thesensing material.
 8. A sensor device as claimed in claim 7 wherein theproperty of interest is a partition coefficient.
 9. A sensor device asclaimed in any preceding claim wherein the stimulus is apharmaceutically active compound or a composition containing apharmaceutically active compound, and the sensing material is adapted tosubstantially eliminate the contribution of dimensional factors tochanges in the effective refractive index of the sensing material.
 10. Asensor device as claimed in any preceding claim wherein the sensingmaterial is in the form of one or more sensing layers capable ofinducing in a secondary waveguide a measurable response to a change inthe localised environment caused by the introduction of or changes inthe stimulus.
 11. A sensor device as claimed in claim 10 wherein thesecondary waveguide is made of silicon oxynitride or silicon nitride.12. A sensor device as claimed in either of claims 10 or 11 furthercomprising an inactive secondary waveguide in which the sensing layer isincapable of inducing a measurable response to a change in the localisedenvironment caused by the introduction of or changes in the stimulus.13. A sensor device as claimed in claim 12 wherein the properties of thesecondary waveguide and inactive secondary waveguide are essentiallyidentical with the exception of the response to a change in thelocalised environment caused by the introduction of or changes in thestimulus.
 14. A sensor device as claimed in claim 12 or 13 wherein thesecondary waveguide and inactive secondary waveguide are made of siliconoxynitride.
 15. A sensor device as claimed in any of claims 1 to 9wherein the sensing material is in the form of a sensing waveguidecapable of exhibiting a measurable response to a change in the localisedenvironment caused by the introduction of or changes in the stimulus.16. A sensor device as claimed in claim 15 further comprising aninactive waveguide substantially incapable of exhibiting a measurableresponse to a change in the localised environment caused by theintroduction of or changes in the stimulus.
 17. A sensor device asclaimed in claim 16 wherein the properties of the sensing waveguide andinactive waveguide are essentially identical with the exception of theresponse to a change in the localised environment caused by theintroduction of or changes in the stimulus.
 18. A sensor device asclaimed in either of claims 16 or 17 wherein the inactive waveguide ismade of silicon oxynitride.
 19. A sensor device as claimed in anypreceding claim wherein the sensing material is an absorbent material ora bioactive material.
 20. A sensor device as claimed in any precedingclaim wherein each of the waveguides of the sensor component is a planarwaveguide.
 21. A sensor device as claimed in any preceding claim whereinthe sensor component constitutes a multi-layered structure.
 22. A sensordevice as claimed in claim 21 wherein the sensor component constitutes alaminate structure.
 23. A sensor device as claimed in claim 21 or 22wherein the multi-layered structure of the sensor component isfabricated onto a silicon substrate and consists essentially of a firstabsorbent layer capable of acting as a sensing layer located above andin intimate contact with a first silicon oxynitride layer capable ofacting as a secondary waveguide, optionally together with one or moreintermediate silicon dioxide layers, wherein said first absorbent layeris of a thickness such that the contribution of dimensional factorsdominate the contribution of compositional factors to changes in theeffective refractive index of the sensing material.
 24. A sensor deviceas claimed in claim 21 or 22 wherein the multi-layered structure of thesensor component is fabricated onto a silicon substrate and consistsessentially of a first absorbent layer capable of acting as a sensinglayer located above and in intimate contact with a first siliconoxynitride layer capable of acting as a secondary waveguide, optionallytogether with one or more intermediate silicon dioxide layers, whereinsaid first absorbent layer is of a thickness such that the contributionof compositional factors dominate the contribution of dimensionalfactors to changes in the effective refractive index of the sensingmaterial.
 25. A sensor device as claimed in claim 21 or 22 wherein themulti-layered structure of the sensor component is fabricated onto asilicon substrate and consists essentially of a first absorbent layerand a second absorbent layer, each of said first and second absorbentlayer being capable of acting as a sensing layer and being located aboveand in intimate contact with a first silicon oxynitride layer capable ofacting as a secondary waveguide, optionally together with one or moreintermediate silicon dioxide layers, wherein said first absorbent layeris of a thickness such that the contribution of compositional factorsdominate the contribution of dimensional factors to the effectiverefractive index of the sensing material and wherein said secondabsorbent layer is of a thickness such that the contribution ofdimensional factors dominate the contribution of compositional factorsto changes in the effective refractive index of the sensing material.26. A sensor device as claimed in any of claims 21 to 25 wherein thefirst silicon oxynitride layer is located above and spaced apart from asecond silicon oxynitride layer capable of acting as a referencesecondary waveguide by an intermediate silicon dioxide layer.
 27. Amethod for measuring a property associated with the introduction of orchanges in a chemical, biological or physical stimulus in a localisedenvironment, said method comprising: providing a sensor device asdefined in any preceding claim; introducing or causing changes in thechemical, biological or physical stimulus in the localised environment;irradiating the sensor component with electromagnetic radiation;measuring the response of the sensor component; and relating theresponse to the property associated with the introduction of or changesin the chemical, biological or physical stimulus.
 28. A method asclaimed in claim 27 wherein the property of interest is swelling of thesensing material.
 29. A method as claimed in claim 27 wherein theproperty of interest is a physicochemical property of the stimulus. 30.A method as claimed in claim 29 wherein the physicochemical property isthe partition coefficient of the stimulus.
 31. A method as claimed inany of claims 27 to 30 wherein the response of the sensor component ismovements in an interference pattern.
 32. A method as claimed in claim31 further comprising: calculating the phase shift from the movements inthe interference pattern; and relating the phase shift to the propertyassociated with the introduction of or changes in the chemical,biological or physical stimulus.
 33. Use of a sensor device as definedin any of claims 1 to 26 for (1) measuring the partition coefficient ofthe stimulus and/or (2) measuring swelling of the sensing material. 34.Use as claimed in claim 33 wherein the stimulus is a pharmaceuticalcompound or a composition containing a pharmaceutical compound.
 35. Aprocess for manufacturing a sensor device as defined in any of claims 1to 26 comprising: (A) obtaining a sensor component including a sensingmaterial capable of inducing or exhibiting a measurable response to achange in the localised environment caused by the introduction of orchanges in the stimulus; (B) arranging the sensor component so as toexpose to the localised environment at least a part of the sensingmaterial; and (C) adapting the sensing material to induce or exhibit amonophasic response to the change in the localised environment caused bythe introduction of or changes in the stimulus.
 36. A process as claimedin claim 35 wherein step (C) comprises: (C1) measuring the specificthickness or thicknesses of sensing material at which a monophasicresponse is induced or exhibited by the sensing material in the sensorcomponent.
 37. A process as claimed in claim 35 wherein step (C)comprises: measuring the thickness of the sensing layer against phasechange over a range of thicknesses; and identifying the specificthickness (or thicknesses) at which a monophasic response occurs.
 38. Amethod for determining the partition coefficient of a pharmaceuticalcompound, said method comprising: (1) providing a first sensor device asdefined in any of claims 1 to 26, wherein said sensing material is apolymer coating; (2) measuring the response of the polymer coating at arange of thicknesses; (3) selecting a specific thickness at which amonophasic response is exhibited or induced by the polymer coating; (4)coating a second sensor device as defined in any of claims 1 to 26 witha polymer coating at the specific thickness; (5) measuring the responseof the second sensor device to the pharmaceutical compound; (6)measuring the responses of the second sensor device to a series ofsamples of known partition coefficient; (7) comparing the response ofstep (5) with the responses of step (6); and (8) determining thepartition coefficient.
 39. An analyser comprising a chromatographicseparation means and one or more sensor devices as defined in any one ofclaims 1 to
 26. 40. An analyser as claimed in claim 39 comprising two ormore sensor devices downstream from a chromatographic separation means,said chromatographic separation means being a column.
 41. An analyser asclaimed in either of claims 39 or 40 wherein the chromatographicseparation means is a high performance liquid chromatographic separationmeans.